1. Field of the Invention
The present invention relates to a liquid ejection apparatus and an ejection abnormality determination method, and more particularly, to pressure abnormality determination technology which measures the pressure inside pressure chambers by means of pressure measurement elements, and detects pressure abnormality on the basis of the measurement results.
2. Description of the Related Art
In a general inkjet recording apparatus (inkjet printer), printing is carried out by ejecting ink droplets at prescribed timings, respectively, from the nozzles of the head, and depositing these ink droplets on a print recording medium, such as recording paper, on the basis of dot pattern data (namely, “dot data” or “print data”) converted from image data for printing inputted from a host computer.
Inkjet recording apparatuses include single-pass printers, which record a desired image on the full surface of the print recording medium, by performing just one scanning action of a line head having a length corresponding to the width of the print recording medium, with respect to the print recording medium, and serial printers, which record a desired image on the full surface of the print recording medium by moving the print recording medium in a direction perpendicular to the breadthways direction of the print recording medium while scanning the print recording medium in the breadthways direction thereof with a head that is shorter than the width of the print recording medium.
In a single-pass method using a line head, if an ejection abnormality occurs in any particular nozzle, then band-shaped non-uniformity occurs in the recorded image, thus causing the quality of the image to decline markedly. The cause of ejection abnormality may be increase in the viscosity of the ink in the vicinity of the nozzles, occurrence of bubbles inside the pressure chambers or inside the nozzles (pressure abnormality in the pressure chambers), adherence of foreign matter (dust, paper particles, etc.) to the nozzles, and the like.
As a method of determining ejection abnormality in the nozzles, there have been proposed a method which determines the ink ejected from the nozzles optically, by means of an optical measurement element, such as a photointerrupter (see Japanese Patent Application Publication No. 9-94959), and a method which determines pressure abnormality on the basis of a pressure measurement signal obtained from pressure measurement elements, and determines ejection abnormality on the basis of these pressure abnormality (see Japanese Patent Application Publication Nos. 2004-284190 and 5-185590), and the like.
In the inkjet recording apparatus and the ink determination method for a recording apparatus described in Japanese Patent Application Publication No. 9-94959, ink droplets ejected from an inkjet head are measured by means of a photointerrupter having a light-emitting element and a light-receiving element, in such a manner that the volume of the ink droplets is measured on the basis of the amount of change in the measurement signal which corresponds to the decrease in the quantity of light reaching the light-receiving element caused by the passage of the ink droplet. However, when the ink droplet volume (dot size) is reduced in order to achieve high image resolution, then the light-emitting elements and light-receiving elements of the optical measurement elements must be disposed at high resolution accordingly, but there are limitations on the degree to which the resolution of the light-emitting elements and light-receiving elements can be raised, and furthermore, it is difficult to determine the cause of any ejection abnormality that may occur and therefore a suitable restoration process cannot be chosen (or implemented).
In the liquid droplet ejection apparatus described in Japanese Patent Application Publication No. 2004-284190, the pressure changes inside the cavities are measured by driving the actuators to a level which does not cause ejection of liquid droplets, in such a manner that ejection abnormality in the liquid ejection head can be determined from the electrical signal thus obtained, on the basis of the temporal change in the capacitance caused by the oscillation of the diaphragm. However, the temporal change in the capacitance caused by the oscillation of the diaphragm varies depending on individual differences within the diaphragm, and drift occurs due to the effects of temperature change, and the like. In order to compensate for this unevenness or drift in the capacitance, a compensatory circuit must be provided in the determination circuit. This compensatory circuit comprises a circuit for converting the capacitance to a frequency, a circuit for converting the frequency to a voltage, a waveform shaping circuit, and the like, and therefore, the circuit composition is complicated.
In the inkjet printer described in Japanese Patent Application Publication No. 5-185590, pressure sensors are provided between the diaphragms and the pressurization mechanisms of the print head, and a control unit is provided for controlling the pressurization mechanisms in such a manner that the pressures applied to the diaphragms by the pressurization mechanisms are uniform, in accordance with the measurement output of the pressure sensors. However, since the unevenness in the capacitance of the pressure sensors is not taken into account, then it may be impossible to accurately measure the movement in the diaphragm, due to the effects of the unevenness in the capacitance of the pressure sensors.